Detect a billing wall once (agent/error_classifier → FailoverReason.billing) and
map it to a recovery link + label in one place, then carry that structured
BillingBlock to every surface instead of re-parsing free-form error text per
surface.
- agent/billing_links.py: provider-agnostic slug/host → (label, billing URL)
table (single source of truth), Nous-aware (is_nous routes to the in-app flow);
unknown providers degrade to a readable label with no invented URL.
- conversation_loop: both billing exit paths return a billing_block through one
helper; the guidance message carries the derived URL for every provider.
- gateway forwards billing_block on message.complete (it was dropped).
- @hermes/shared: BillingBlock type shared by desktop + TUI.
